The debate about why it is so difficult to get the governments of rich and powerful countries to do anything effective about massacre, persecution and genocide raged with especial intensity throughout the 90s.

What were the mechanisms of denial that allowed political leaders to procrastinate in the face of such plainly evil developments, and why, when action was in some cases finally taken, was intervention so clumsy and the restoration of damaged societies so often mismanaged?

These two books - one a historical study of American policy since the first world war and the other a scrappy but vivid bouquet of reportage on recent United Nations operations - speak passionately to such questions.

As the failures of the past decade unfolded, aid workers, soldiers, journalists, academics and politicians agonised over what was going wrong. They cursed and despaired on muddy Balkan roads and in African refugee camps, they argued in scruffy hotels in besieged Sarajevo, they weighed their experiences in officers' messes, university common rooms, foreign ministries and NGO headquarters back in Europe and the US. A few angry young American diplomats even sat down at their desks in Washington and wrote letters of resignation. These were men and women who had expected that the end of the cold war would clear the way for serious international action of all kinds, including military action if need be, whenever and wherever it was required. What they saw instead were the stupidities of the early intervention in Bosnia, the disaster in Somalia, the disgrace of Rwanda and the farce of Haiti.

Many have since written books, some simply recording the often dreadful immediacies of their experience, others trying to stand back to set these failures in a broader historical context. Samantha Power, who was a correspondent in the Balkans and is now executive director of the Carr Centre for Human Rights at Harvard, has produced one of the best works in the latter category.

Linda Polman's book belongs in the former and is an altogether lighter effort, recasting intervention as black comedy, but it has its telling moments. The description in her final chapter of a manic 24 hours with Zambian UN troops in a place called Kibeho in Rwanda is a nightmare scene from the theatre of the absurd to which peacekeeping so often belongs. Alternately funny and harrowing, it shows what can happen when the powers insist that a UN force be dispatched but refuse to provide it with the men, machines or mandate required to succeed.

Power's starting point is not the United Nations or military intervention in general, but the concept of genocide. Thus the destruction of Armenian society in Turkey, the Holocaust, then Cambodia and then Iraq in the 80s precede her survey of more recent events. What her study shows is that victory in the field of ideas does not necessarily lead to victory in the real world.

Raphael Lemkin, the refugee Polish Jew who almost singlehandedly forced the idea of genocide on to the world agenda and whose driven quality Power brilliantly evokes, believed that once genocide was recognised in international and national law, it would inevitably be the more forcefully opposed. But this turned out to be true only in a limited sense. What happened instead was that governments, and especially the US government, saw that using the word "genocide" might well commit them to action, while not using it would allow them to avoid action.

The shameful twisting and turning that resulted was seen at its worst in the State Department's consistent refusal to apply the word to the massacre of 800,000 people in Rwanda in 1994. Earlier there had been similar slipperiness over killings in Cambodia at a time when the United States was more concerned to punish the Vietnamese than to recognise the enormity of what the Khmer Rouge had done. Yet, when Washington and its allies wanted to bomb Serbia to force its withdrawal from Kosovo, they had no difficulty finding the word in the dictionary.

Power concludes that "the United States has consistently refused to take risks to suppress genocide", and on occasion has supported genocidal regimes, as it did with the Khmer Rouge regime after the Vietnamese invasion and with Saddam's regime after its Anfal campaign against the Kurds. The extraordinary 40-year effort to get the convention on genocide ratified in Congress showed the US legislature betraying the same reluctance as the executive to be bound by a word. It got through in the end, in emasculated form, only because Ronald Reagan needed to do something to counterbalance his ill-judged visit to Bitburg and its SS graves.

Power reminds us that the American right - and not only the right - has a long history of refusing any arrangement under which the US can be judged by foreigners, or made to take action because of decisions by foreigners. The antecedents of the Bush administration's vendetta against the international criminal court, itself essentially a product of the campaign to prevent genocide which the US has rhetorically supported, are clear to see.

President Clinton watched two genocides unfold, one in the Balkans and another in Rwanda. While America eventually acted in Bosnia and Kosovo, Clinton had to be pushed in each case, and in each case his decisions were largely informed not by considerations of what was right or wrong but by his sense of his own political standing.

"I'm getting creamed," he complained when Senator Bob Dole's skewering of the president's inertness over Bosnia finally began to lose him some political support. On Rwanda, the United States not only did nothing, but it prevented other countries and the UN from doing anything, manoeuvring, with Britain, to reduce the US military mission in Rwanda from a grossly inadequate to an utterly token level.

Power is good at analysing the vicious circle that inhibits intervention in such tragedies. The US government will not act because it perceives no public support for action, the military is opposed for the same reason, and politicians sense they can undermine their opponents by portraying them as obsessed with the problems of violent and probably deranged foreigners. Congressman Frank McCloskey, for example, who harried the Clinton administration over its failures in the Balkans, lost his seat after the Republicans accused him of being "more concerned about Bosnia than Evansville", the main town in his district.

Yet a confident government, convinced of the rightness of its policies, can, by informing, arguing with and propagandising its citizens, create a constituency for the actions it wants to take. That course, not taken by the Clinton administration to build support for interventions in the Balkans and Africa, was taken by the second Bush administration after it decided on intervention in Iraq.

Power's book was written before the Iraq war and was probably largely completed before September 11 2001; Polman's was first published in Dutch in 1997. Neither, therefore, can take much notice, beyond a mention in a preface or endpiece, of these recent events. The Bush administration was of course aided by the shift in public consciousness caused by the al-Qaida attacks.

We have yet to work out properly how the post-twin towers interventions relate to those that went before. But there is obvious irony in the fact that while previously, as these books illustrate so clearly, determination was often lacking to deal with crises that most people agreed were serious, there was no shortage of it when the Bush administration moved to deal with a crisis on which there was no global consensus at all.